1. Field of the Invention
The present invention relates to a technique for treating water to be treated and more particularly, to an effective technique for washing a water quality sensor that detects the water quality.
2. Description of the Related Art
According to known water treatment apparatus for treating water, water quality is detected by using a water quality sensor. For example, Japanese laid-open patent publication No. 3-52696 discloses a water treatment apparatus of this type in which a DO sensor is provided in an aeration tank disposed upstream of a sterilizing tank in a submerged state. The quality of the aerated water can be kept track of by the DO sensor.
The water quality sensor is submerged in a purifying region to detect the water quality and therefore, an accuracy of water quality detection tends to be impaired by deposition of biomembrane on the sensor surface. In this connection, the water quality sensor is preferably installed particularly in a downstream region in which aerobically treated water flows. However, if the water quality sensor is installed downstream of the aerobic treatment region, sludge is generated due to aerobic treatment and deposited on the sensor surface. Therefore, it becomes difficult to enhance the accuracy of water quality detection to a desired level.